Ambient-pressure organic superconductor

ABSTRACT

A new class of organic superconductors having the formula (ET) 2  MX 2  wherein ET represents bis(ethylenedithio)-tetrathiafulvalene, M is a metal such as Au, Ag, In, Tl, Rb, Pd and the like and X is a halide. The superconductor (ET) 2  AuI 2  exhibits a transition temperature of 5 K which is high for organic superconductors.

CONCEPTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. W-31-109-ENG-38 between the U.S. Department of Energy and TheUniversity of Chicago representing Argonne National Laboratory.

This is a division of application Ser. No. 738,808 filed May 29, 1985.

BACKGROUND OF THE INVENTION

This invention relates to electrical conductors based on organicstructures and more particularly to organic superconductors withimproved transition temperatures at ambient pressure.

Electrical conductors based on organic structures and particularlyorganic superconductors represent a relatively new area of developmentcompared to the metallic counterparts. While some metallicsuperconductors such as niobium and alloys of niobium have been usedcommercially for fabricating coils for supermagnets, organicsuperconductors are in a relatively early stage of development. Organicsuperconductors have been in general referred to as organic metals orsynthetic metals because they have metal-like electrical conductivitywhich does not derive from the electrons of metal atoms. They also maybe varied in structure by changes in composition and in general have alower density than the metallic superconductors.

The first organic superconductor was (TMTSF)₂ X where TMTSF is theselenium-based organic donor tetramethyltetraselenafulvalene and X is acomplex univalent anion such as ClO₄ ⁻ or PF₆ ⁻. The superconductingproperties of these salts were first observed in 1980. These salts arenow identified as Bechgaard salts.

In general, the Bechgaard salts undergo metal-to-insulator transitionsas temperatures are decreased. For some of the Bechgaard salts, thesetransitions may be suppressed by applied pressures of several thousandsof atmospheres to achieve a superconducting state near 1 K. In 1981,Bechgaard and coworkers reported that (TMTSF)₂ ClO₄ exhibitsambient-pressure superconductivity with a transition temperature ofabout 1.2 K.

A more recent cation donor which produces organic superconductors isBEDT-TTF or ET which represents bis(ethylenedithio)tetrathiafulvalene.With ClO₄ ⁻ as the anion in (ET)₂ ClO₄ (C₂ H₃ Cl₃)₀.5, the materialreportedly has metallic conductivity but is not superconducting down toT=1.4 K. (ET)₂ ReO₄ is reportedly superconducting at 4000 atmosphereswith a transition temperature of about 2 K. When the anion (I₃ ⁻) wassubstituted by Russian researchers to produce β-(ET)₂ I₃,superconductivity at ambient pressure was achieved near 1.4 K.Subsequently, β-(ET)₂ IBr₂ was prepared which exhibits a transitiontemperature of 2.7 K.

One difference between these compositions was the reduction by about 7%in the total length of the anion (trihalide). This difference wasexpected to decrease the size of the unit cell in β-(ET)₂ IBr₂ andincrease the sulfur atom orbital overlap. The orbital overlap in thesulfur atom network constitutes the conductive band in these ET-basedorganic metals. Arguments based on increased overlap would predict alower transition temperature for β-(ET)₂ IBr₂. When a higher temperaturewas observed, the results were somewhat surprising. Subsequently, in thehope of achieving a higher T_(c), a smaller anion derivative, (ET)₂ICl₂, was synthesized but exhibited no superconducting properties.

While the above work has provided significant results, it is ofparticular importance to increase transition temperatures to at leastabove the boiling temperature of helium (4.2 K). Helium could then beused to control the operating temperature of the organic superconductor.

Accordingly, one object of the invention is new organic superconductors.A second obJect of the invention is organic superconductors withincreased transition temperatures. Another object of the invention isorganic superconductors with a transition temperature above 4.2 K. Theseand other objects will become apparent from the following description.

SUMMARY OF THE INVENTION

Briefly, the invention relates to new organic superconductors having theformula (ET)₂ MX₂ wherein ET representsbis(ethylenedithio)tetrathiafulvalene, M is a metal, X is a halide andthe metal is selected so that the anion length of MI₂ is between that ofI₃ and IBr₂. Suitably, the metal is Au, Ag, In, Tl, Rb, Pd and the likeand preferably is Au. The halide includes iodide, bromide and chlorideand preferably is iodide. Further, MX₂ preferably is symmetrical such asI-M-I. The composition β-(ET)₂ AuI₂ has been found to have a transitiontemperature of 5 K which may be the first ambient pressure organicsuperconductor with a transition temperature as high as 5 K.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is directed to a class of organic compositions representedby the formula β-(ET)₂ MX₂ wherein ET isbis(ethylenedithio)tetrathiafulvalene, M is a metal, X is a halide andthe anion length of MI₂ is between that of I-I-I and Br-I-Br. The β formof (ET)₂ MX₂ is used and it is also preferred that the anion besymmetrical. The metal includes Au, Ag, In, Tl, Rb, Pd and the like andpreferably is Au. The halide has an atomic number in the range of 17-53such as iodide, bromide and chloride and preferably is iodide. Otherpreferred compounds are: (ET)₂ InI₂, (ET)₂ TlI₂, (ET)₂ TlBr₂, (ET)₂TlCl₂ and (ET)₂ AgI₂. Particularly preferred is β-(ET)₂ AuI₂. Anionlengths for determining the atom combinations may be determined in suchreferences as Inorganic Chemistry, James E. Huheey, Harper andRow-Publishers, (1972) pp. 184-185.

The procedure used to prepare (ET)₂ MX₂ involves two steps: the chemicalsynthesis of n-Bu₄ NMX₂ and the electrocrystallization of (ET)₂ MX₂ byusing ET as an organic donor and n-Bu₄ NMX₂ as supporting electrolyte.The anion MX₂ ⁻ is synthesized by refluxing MX with n-Bu₄ NX. Forexample, when M=Ag, X=Br and I, n-Bu₄ NAgBr₂ and n-Bu₄ NAgI₂ areprepared in this fashion. A similar procedure is described in ActaChemica Scandinavica A28, (1974) p. 255.

The electrocrystallization is carried out in an H cell A solutioncontaining ET(1˜mM) is prepared in a dry box and added to the anodecompartment of the H cell. Similarly, a solution containing n-Bu₄ NMX₂(10˜50 mM) is prepared and used to fill the cathode compartment and toequalize fluid levels on both sides. Cleaned and dried electrodes areinserted in the dry box. Crystals are grown at a constant current(0.5˜2.0 μA) and constant temperature (22˜±0.2° C.) environment. Blacklustrous compounds of (ET)₂ MX₂ are harvested from anode after on tofour weeks of growth. Detailed procedure for electrocrystallization isreported in Inorganic Syntheses, Vol. 24, Ed. J. M. Shreeve.

As with other superconductors, their use will include a layer of aconductive metal such as copper, silver and the like. Typically, eachlayer is of a thickness sufficient to carry the desired levels ofcurrent with 400-1000 Å being the superconductor and 1000-2000 Å for theconductive metal. The organic superconductor may be formed on thesupporting metal by various types of deposition techniques such asprecipitation and the like.

In testing for transition temperature, an inductive test is used todetermine the temperature at which a resonance frequency is changed inan electrical coil. As reported in Physical Review B, Vol. 30, Number 5,(1984) pp. 2958-2960, rf penetration depth measurements are carried outto determine transition temperatures.

The rf penetration depth measurements are carried out on crystals cooledin a pumped liquid-helium (He⁴ or He³) cryostat surrounded by asuperconducting magnet. The samples are contained in epoxy cylinderswhich could be inserted in an rf coil consisting of approximately threehundred turns of copper wire wound on an epoxy coil form measuring 3.75mm in length and 2.05 mm in diameter. The coil is operated atfrequencies of ˜513 kHz. The rf penetration depth is measured attemperatures down to 0.44 K and in fields up to approximately 15 kOe.Temperatures are determined from the vapor pressure above the liquid He³or He⁴. Superconductivity is detected by the increase in the resonantfrequency of the rf coil caused by the exclusion of the rf field fromthe sample by persistent shielding currents. Because changes in the rffrequency of 1 part in 10⁵ can be detected, this technique has a highsensitivity to superconductivity in the relatively small samplesavailable for the experiment.

The following example is provided for illustrative purposes and is notintended to be restrictive as to the scope of the invention:

EXAMPLE I

β-(ET)₂ AuI₂ was prepared and tested for superconducting properties.

Lustrous black crystals of β-(ET)₂ AuI₂, where ET isbis(ethylenedithio)tetrathiafulvalene as shown by the formula ##STR1##were grown by electrocrystallization by the use of 9.3 mg of ET (StremChemicals, Inc., 1.6 mM) as organic donor and 157 mg of n-Bu₄ NAuI₂(15.1 mM) as supporting electrolyte at 23° C. in a standard 15 mLcapacity H-cell. The anion and the supporting electrolyte were preparedfollowing the procedure involving the replacement of Br⁻ by I⁻ in n-Bu₄NAuBr₂ which is described in J. Chem. Soc. Dalton Trans. (1973) p. 1845.Its purity was confirmed by its mp (78°-79°) and elemental analysis.Anal. calcd. (found) for n-Bu₄ NAuI₂ : C, 27.72 (27.68); H, 5.23 (5.32);N, 2.02 (1.96); I, 36.61 (36.83). In the preparation, dry THF was usedas solvent, and a 1.0 μA/cm² current density was applied. Crystalformation was observed within 24 hours and the fully grown distortedhexagons were harvested after about one week. The β-(ET)₂ AuI₂ crystalsare characterized by their room temperature peak-to-peak ESR linewidthof ˜20 gauss.

Single crystal X-ray analysis revealed that β-(ET)₂ AuI₂ is clearlyisostructural [space group pT, V_(c) =845.2(3)Å³, Z=1] with β-(ET)₂ X,X=I₃ ⁻ and IBr₂ ⁻. The unit cell volumes for β-(ET)₂ X, X=I₃ ⁻ and IBr₂⁻ are 855.9(2) Å³ and 828.7(3) Å³, respectively which indicates that, asexpected, the (I-Au-I)⁻ anion is of intermediate length between those ofthe two trihalide anions. Crystal structure studies show that the anionlengths are ˜10.2 Å for the I₃ ⁻ anion, ˜9.3 Å for the IBr₂ ⁻ anion, and˜9.4 Å for the AuI₂ ⁻ anion. The structure consists of discrete layersof AuI₂ ⁻ anions between which is sandwiched a "corrugated sheetnetwork" of ET molecules with short (d_(s) . . . s ≦3.60 Å the van derWaals radius sum for S) interstack S . . . S interactions. The looselypacked stacking of the ET entities is characterized by intrastack S . .. S distances exceeding 3.60 Å.

The superconducting state in β-(ET)₂ AuI₂ was detected by rf penetrationdepth measurements at ambient pressure and at various applied magneticfields, similar to measurements previously reported for the I₃ ⁻ andIBr₂ ⁻ derivatives.

When tested for superconductivity, the sample of three relatively largecrystals gave an apparent onset temperature for bulk superconductivity(T_(c)) of 4.97±0.06 K, which is a very high T_(c) observed at ambientpressure for an organic superconductor. Measurements of the onsettemperatures of the individual crystals gave T_(c) =3.93±0.04 K,4.36±0.04 K, and 4.98±0.08 K, respectively.

The foregoing description of embodiments of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A superconductorcomprising a first layer of a compound with superconductor propertieshaving the formula (ET)₂ MX₂ where ET isbis(ethylenedithio)tetrathiafulvalene as shown by the formula ##STR2##where M is a metal, X is a halide, and MX₂ is symmetrical; and a secondsupporting layer of a conductive metal.
 2. The superconductor of claim 1wherein M is Au, Ag, In, Tl, Rb, or Pd and X is I, Br or Cl.
 3. Thesuperconductor of claim 2 wherein the second layer is Cu.
 4. Thesuperconductor of claim 3 wherein M is Au and X is I.